(1) Field of the Invention
The present invention relates to fiber optic sensor arrays and, more particularly, to a thermal compensated fiber Bragg grating package for use in a fiber optic sensor array.
(2) Description of the Prior Art
The use of fiber optic sensors to determine the presence or magnitude of an external parameter is generally known in the art. A fiber optic hydrophone typically includes a hollow compliant mandrel with an optical fiber wound on the mandrel surface to sense pressure generated by acoustic pressures under water. One type of wavelength and time division multiplexed (WDM/TDM) fiber optic hydrophone array is based upon fiber Bragg gratings (FBGs). FBGs are periodic refractive index variations infixed into the core of the optical fiber by an ultraviolet laser. In the sensor array, the gratings serve as reflectors implementing the interferometric sensor. A typical array includes a plurality of FBGs separated by lengths of sensor fiber wound on the compliant sensor mandrels.
The FBGs reflect over a very narrow wavelength band, and it is desirable to minimize the shift of this band with external factors such as pressure and temperature. When such spectral shifts are minimized, multiple wavelength channels can be placed closer in wavelength, allowing the system to have more sensor channels at different wavelengths. Normally, the FBG1s thermal sensitivity is dominated by the thermo-optic effect on the fibers index of refraction. If, however, the FBGs were merely wound with the rest of the fiber on the compliant plastic mandrels of the hydrophone, the large thermal expansion coefficient of the plastic would dominate and lead to excessive thermal shift of the gratings. Additionally, the sensor mandrels are specifically designed to be sensitive to pressure, and the depth changes of a towed hydrophone array would also cause significant changes in the FBGs if wound directly on the mandrel.
Research has been performed on temperature insensitive packages for FBGs, for example, as described in the article entitled, “Temperature Compensation Technique for Fiber Bragg Gratings Using Liquid Crystalline Polymer Tubes,” T. Iwashima et al., Electronic Letters, Volume 33, No. 5, 27 Feb. 1997, pp. 417-418, incorporated herein by reference. The package described in this article makes use of a material with a negative coefficient of thermal expansion to offset the main thermo-optic effect in the grating. The optical fiber with the grating is fixed inside a linear structure comprised of a tube made of the negative expansion material.
One problem with this type of FBG package and other types of FBG packages is that they are unsuitable for automated hydrophone array fabrication because they are linear structures. This type of linear or straight FBG package cannot be wound around a thin, cylindrical towed array mandrel. In order to use this type of linear FBG package, the winding of an array must be interrupted, the FBG must be placed into the linear package, and the package must be positioned by hand along the axis of the array. The machine winding of the next array portion can then commence. Since the existing linear FBG packages must be placed longitudinally, the length of the sensor is increased to accommodate the length of the linear structure.
This type of FBG package also cannot be packaged ahead of time because the straight packages cannot go through the pulleys on a winding machine. When a grating is to be placed, the completed winding must be tacked down with epoxy so that it does not spring loose when fiber is pulled off the winder by hand to package and lay the FBG. Thus, the existing linear FBG packages cannot be used in an automated array assembly.